1. Field of the Invention
The present invention generally relates to a filtering device and a circuit module, and particularly, to a filtering device and a circuit module using a distributed constant circuit.
2. Description of the Related Art
The UWB (Ultra-Wide-Band) communication scheme is attracting attention in short-distance radio communications. Generally, UWB communication indicates communications which utilizes a frequency band higher than 500 MHz or a frequency band having a band ratio higher than 20%, carries out digital modulation and direct spreading to a high frequency band, and thereby allows utilization of a frequency band as wide as a few GHz and radio communications at speed as high as a few Mbps.
In UWB communications, in order that existing electromagnetic signals are not interfered with during wide-band communications, a wide-band and sharp band-pass filter is required.
The existing dielectric filters, or SAW (surface acoustic wave) filters, however, only have band ratios lower than 8%, and it is thought that further expansion of the band ratios is difficult.
To solve this problem, development is made of a ring filter using a distributed constant circuit in order to obtain wide-band frequency characteristics. For example, Japanese Laid-Open Patent Application No. 7-183732 and Japanese Laid-Open Patent Application No. 11-17405 disclose techniques in this field.
Because the ring filter is a distributed constant circuit, is can be constructed in a plane, and is able to obtain wide pass-band, low-pass loss, and a sharp attenuation pole. For these reasons, attention is being paid to application of the ring filter to UWB communications.
FIG. 1 is a view illustrating a structure of the ring filter.
As illustrated in FIG. 1, a ring filter 1 includes a ring portion 11 and an open stub 12. The ring portion 11 includes a λ/2 path portion 11a, a first λ/4 path portion 11b, and a second λ/4 path portion 11c. Here, λ represents the wavelength corresponding to a central frequency.
One end of the λ/2 path portion 11a is connected to a port P1, and the other end of the λ/2 path portion 11a is connected to a port P2.
One end of the first λ/4 path portion 11b is connected to the port P1, and the other end of the first λ/4 path portion 11b is connected to one end of the second λ/4 path portion 11c. 
One end of the second λ/4 path portion 11c is connected to the first λ/4 path portion 11b, and the other end of the second λ/4 path portion 11c is connected to the port P2.
One end of the open stub 12 is connected to the connecting point of the first λ/4 path portion 11b and the second λ/4 path portion 11c, and the other end of the open stub 12 is open.
FIG. 2 shows pass-band characteristics of the ring filter.
Using the ring filter 1 illustrated in FIG. 1, it is possible to obtain band-elimination characteristics as shown in FIG. 2, namely, two attenuation pole frequencies f1, f2 are symmetrically located on two sides of the central frequency, which is defined to be the frequency f0 corresponding to the wavelength λ.
However, the ring filter 1 showing the band-elimination characteristics as shown in FIG. 2 cannot be used as a band-pass filter directly, because the frequency attenuation poles are too sharp.
Upon that, it is proposed to expand the low-frequency attenuation poles and the high-frequency attenuation poles of plural ring filters, and connect these ring filters in cascade so as to expand the bands of the low-frequency attenuation pole and the high-frequency attenuation pole, and obtain frequency characteristics close to that of a band-pass filter. For example, this technique is described by Ishida et al., in “Development of wide-band ring filter”, Technical Report of IEICE, WBS2003-20, MW2003-32 (2003-05).
FIG. 3 is a view of a filtering device using the ring filters.
FIG. 4 shows the band characteristics of the filtering device using the ring filters.
As illustrated in FIG. 3, a filtering device 20 includes a first ring filter 21, a second ring filter 22, and a third ring filter 23.
The first ring filter 21, the second ring filter 22, and the third ring filter 23 have the same structure as shown in FIG. 1. One end of the first ring filter 21 is connected to the port P1, and the other end of the first ring filter 21 is connected to the second ring filter 22. One end of the second ring filter 22 is connected to the first ring filter 21, and the other end of the second ring filter 22 is connected to the third ring filter 23. One end of the third ring filter 23 is connected to the second ring filter 22, and the other end of the third ring filter 23 is connected to the port P2.
The first ring filter 21 includes an open stub 21a, a λ/2 path portion 21b, λ/4 path portion 21c, and λ/4 path portion 21d, and widths and lengths of the open stub 21a, the λ/2 path portion 21b, the λ/4 path portion 21c, and the λ/4 path portion 21d are specified such that the first ring filter 21 shows frequency characteristics having two attenuation pole frequencies f11 and f12, as shown by the dashed line in FIG. 4. With given widths and lengths of the open stub 21a, the λ/2 path portion 21b, the λ/4 path portion 21c, and the λ/4 path portion 21d, the impedances of the open stub 21a, the λ/2 path portion 21b, and the λ/4 path portions 21c, 21d are uniquely determined, and are denoted as Z11, Z12, and Z13, respectively.
The second ring filter 22 includes an open stub 22a, a λ/2 path portion 22b, and λ/4 path portions 22c, 22d, and widths and lengths of the open stub 22a, the λ/2 path portion 22b, and the λ/4 path portions 22c, 22d are specified such that the second ring filter 22 shows frequency characteristics having two attenuation pole frequencies f21 and f22, as shown by the dot-dashed line in FIG. 4. The corresponding impedances of the open stub 22a, the λ/2 path portion 22b, and the λ/4 path portions 22c, 22d are determined to be Z21, Z22, and Z23.
Similarly, the third ring filter 23 includes an open stub 23a, a λ/2 path portion 23b, and λ/4 path portions 23c, 23d, and widths and lengths of the open stub 23a, the λ/2 path portion 23b, and the λ/4 path portions 23c, 23d are specified such that the third ring filter 23 shows frequency characteristics having two attenuation pole frequencies f31 and f32, as shown by the double dot-dashed line in FIG. 4. The corresponding impedances of the open stub 23a, the λ/2 path portion 23b, and the λ/4 path portions 23c, 23d are determined to be Z31, Z32, and Z33.
The frequency characteristics of the filtering device 20 correspond to a combination of the frequency characteristics of the first ring filter 21, the second ring filter 22, and the third ring filter 23, and are shown by the solid line in FIG. 4. As shown in FIG. 4, by connecting the first ring filter 21, the second ring filter 22, and the third ring filter 23 in cascade, which have different low-frequency attenuation poles and high-frequency attenuation poles, the bands of the low-frequency attenuation pole and the high-frequency attenuation pole of the filtering device 20 are expanded, as shown by the solid line in FIG. 4, resulting in frequency characteristics close to those of a band-pass filter.
In the above descriptions, for simplicity, it is assumed that three ring filters are connected in cascade, however, in practical UWB communications, three-stages of ring filters are not sufficient, and a larger number of stages of ring filters is needed. However, when connecting more ring filters in cascade, the size of the filtering device increases, and pass loss in the filtering device increases.